1. Field of the Invention
This invention relates to a phase change optical recording medium and a method for preparing the same.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected. Optical recording media of the phase change type are of great interest since they can be overwritten by modulating the intensity of a single light beam and the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of the phase change type used Ge-Te systems which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was recently proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds were investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228.
Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai (JP-A) No. 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A 267192/1992, 232779/1992, and 166268/1994 disclose phase change type optical recording media wherein an AgSbTe.sub.2 phase forms when a recording layer crystallizes.
For prior art phase change type optical recording media, recording layers are formed using vacuum deposition equipment and remain amorphous immediately after formation. The recording layers must be crystallized by an operation generally known as initialization before the recording media can be utilized as rewritable media.
Initialization is carried out in various ways, for example, after a recording layer is formed on a substrate, by heating the substrate to the crystallization temperature of the recording layer for crystallization as disclosed in JP-A 3131/1990; irradiating a laser beam to the recording layer for crystallization, which method is called solid phase initialization, as disclosed in JP-A 366424/1992, 201734/1990 and 76027/1991; irradiating flash light to the substrate to achieve pseudo-crystallization by so-called photo-darkening, which method takes advantage of the photo characteristics of calcogen compounds, as disclosed in JP-A 281219/1992; and high-frequency induction heating the medium. JP-A 98847/1990 proposes to heat a substrate during formation of a recording layer to thereby crystallize the recording layer. JP-A 5246/1990 discloses a method involving the steps of forming a first dielectric layer, forming a recording layer thereon, heating it for crystallization, and forming a second dielectric layer thereon.
However, the initialization step by laser beam irradiation takes a long time and causes low productivity. Heating of the overall medium rejects the use of inexpensive resin substrates. That is, resin substrates can be distorted upon heating for initialization, causing tracking errors. The method of irradiating flash light is also low in productivity because several shots of irradiation are necessary to achieve full crystallization.
Under the circumstances, the use of a so-called bulk eraser is the only technique which is regarded commercially acceptable and currently used. The bulk eraser irradiates a beam from a high power gas or semiconductor laser through a relatively large aperture stop for crystallizing a multiplicity of tracks altogether. Since the bulk eraser permits the recording layer to be locally heated, the substrate temperature is elevated to a little extent, enabling the use of less heat resistant resins as substrates.
The bulk eraser, however, requires a time of several minutes for initializing optical recording discs of 12 cm in diameter. Then the initializing step is a rate-determining step in the making of optical recording discs. While TeGeSb base materials are currently most widely used for phase change recording layers, it is believed that the initializing operation cannot be removed insofar as these materials are used.
Prior art phase change type recording media require to repeat rewriting several times after initialization until a constant rate of erasure is reached. In most cases, rewriting is repeated about ten times before performance rating is carried out. The reason why the rate of erasure remains unstable upon rewriting immediately after initialization is that the formation of a AgSbTe.sub.2 or In--Te crystalline phase is incomplete.
To eliminate the initialization step which is required by prior art phase change type recording media, U.S. Ser. No. 08/598,913, entitled "Method for Preparing Phase Change Optical Recording Medium" and assigned to the same assignee as the present invention, proposes a method for forming a In--Ag--Te--Sb base recording layer by separately effecting the step of sputtering Sb+In and the step of sputtering Ag+Te or by separately effecting the step of sputtering Sb, the step of sputtering In, and the step of sputtering Ag+Te. The recording layer formed by such a series of steps has been at least partially crystallized. After recording is repeated on the recording layer formed by this method so that the elements in the recording layer are fully diffused and mixed with each other, a sufficient change of reflectance is obtained as acquired after initialization by the bulk eraser. However, in the duration from immediately after the formation of the recording layer to several times of rewriting, the rate of erasure remains unstable like prior art phase change type recording media. More particularly, since reflectance is different between the region crystallized during formation and the region crystallized upon rewriting, the reflectance remains unstable until the rewritten regions are extended by increments throughout the entire surface of the recording layer. In the case of mark edge recording utilized in rewritable digital video discs (DVD-RAM), such reflectance variations can be mistaken for mark edges.
JP-A 106647/1996 discloses a phase change type recording medium comprising a recording layer in the form of AgInSbTe system artificial superlattice film having alternately deposited AgSbTe.sub.2 films and In--Sb films or having alternately deposited AgSbTe.sub.2 films, In films, and Sb films. One of the alleged advantages is that the initialization energy required for the entire recording layer is reduced because the crystallized AgSbTe.sub.2 films are used.
We found that when an AgSbTe.sub.2 film and an In--Sb film were stacked, as in the case of U.S. Ser. No. 08/598,913, the reflectance remained unstable in the duration from immediately after the formation of the recording layer to several times of rewriting. Also when an AgSbTe.sub.2 film, an Sb film, and an In film were alternately deposited, the reflectance remained unstable until rewriting was done several times. To acquire a stable reflectance in the crystalline region upon rewriting, the In--Te crystalline phase must be present in the crystalline region. However, in the embodiment of JP-A 106647/1996 wherein indium is not present in the AgSbTe.sub.2 film, but as the In--Sb film or In film, it becomes difficult for indium to bond with tellurium to form an In--Te crystalline phase. Where initialization is carried out with low energy as described in JP-A 106647/1996, the In--Te crystalline phase cannot be fully formed during the initialization. For this reason, the reflectance remained unstable until the In--Te crystalline phase is fully formed by repeating rewriting several times. It is noted that specific initializing conditions such as linear velocity and laser power are described nowhere in JP-A 106647/1996.
In examples described in JP-A 106647/1996, the Sb films and In--Sb films have a thickness of less than 5 nm. These films cannot be crystalline when their thickness is less than 5 nm. As a result, the reflectance of the recording layer immediately after its formation is very low. Since the low reflectance hinders focusing of a laser beam and hence, uniform heating, it becomes difficult to achieve uniform initialization.
Still further, the content of indium in the In--Sb film is described nowhere in JP-A 106647/1996. In Example of JP-A 106647/1996, a laminate construction wherein indium and antimony are separated into In films and Sb films is simply compared with a single layer construction wherein indium and antimony are not separated, but formed into an In--Sb film. It is thus believed that the composition of the In--Sb film is the same as the combination of In and Sb films. Since the In and Sb films have the same gauge, it is believed that the indium content in the In--Sb film is about 10 to 15 at %. Such a high indium content makes it difficult to form an In--Sb film as a crystalline one even if its thickness is increased. There still arises the above-mentioned problem associated with initialization.